Aristarchus of Samos

Lore & Background

He likely moved to Alexandria and became a student of Strato of Lampsacus, who later headed the Peripatetic school in Greece. Vitruvius records that he built two types of sundials: a flat disc and a hemispherical one. His only known surviving work, On the Sizes and Distances of the Sun and Moon, uses a geocentric framework for geometric calculations but does not present or endorse a geocentric cosmology. The reasoning that the Sun's greater size made it the natural center of the universe is an inference not directly stated in his surviving writings. He proposed that the fixed stars were extremely distant, explaining why no stellar parallax was observable. The order of planets in his heliocentric model is not recorded, and while some have speculated about influence from Philolaus of Croton's concept of a central fire, direct influence is not established canon. A common misconception holds that Aristarchus was persecuted for his heliocentric views. According to Lucio Russo, this error stems from Gilles Ménage's misprint of a passage from Plutarch, which reversed the subject and object so that Aristarchus appeared to be accused of impiety rather than joking about it. In reality, the heliocentric model was not considered sacrilegious by his contemporaries, though it was later rejected in favor of the geocentric theories of Aristotle and Ptolemy.

Reader's Guide

Aristarchus of Samos, active in the third century BC, is best known for advancing the first recorded heliocentric model, placing the Sun rather than the Earth at the center of the cosmos. He proposed that the Earth orbits the Sun once yearly and rotates on its axis daily. This idea built upon Philolaus of Croton’s concept of a central fire, which Aristarchus reinterpreted as the Sun, and he correctly ordered the other planets by distance around it. He also supported Anaxagoras’s view that the Sun was merely another star. His reasoning was partly based on his estimate that the Sun was about seven times larger than Earth (the actual figure is 109 times in diameter), leading him to conclude that the larger body should naturally be the center. Aristarchus’s heliocentric hypothesis was not widely accepted in antiquity; geocentric models from Aristotle and Ptolemy dominated. However, Archimedes preserved the theory in *The Sand Reckoner*, noting that Aristarchus placed the fixed stars at an immense distance to explain the lack of observable stellar parallax. A common misconception that Aristarchus was persecuted for impiety stems from a later misprint in Plutarch’s text, where an accusative and nominative were swapped, making it appear that he, rather than the Stoic Cleanthes, was accused. Aristarchus also made geometric attempts to measure the sizes and distances of the Sun and Moon. His only surviving work, *On the Sizes and Distances of the Sun and Moon*, uses a geocentric framework and an inaccurate 87° angle at half moon to conclude the Sun is 18 to 20 times farther than the Moon (the true value is about 400 times). He noted that the Moon and Sun have nearly equal apparent sizes, so their diameters must be proportional to their distances. His solar parallax estimate, though imprecise, was used by astronomers until Tycho Brahe. The heliocentric theory was later revived by Copernicus, refined by Kepler, and explained by Newton.

Did You Know?

• He built two types of sundials: a flat disc and a hemispherical one.
• A misprint in a 17th-century edition of Plutarch wrongly made Aristarchus appear accused of impiety for his heliocentric views.
• His only surviving work, On the Sizes and Distances of the Sun and Moon, uses a geocentric framework for calculations but does not endorse a geocentric cosmology.